1. Field of Art
The present invention relates to organic nonlinear optical materials and nonlinear optical devices in which nonlinear refractive indices of the organic nonlinear optical materials are utilized.
2. Prior Art
Third-order nonlinear optical materials attract attention as important materials for optical devices in the future because they exhibit frequency conversion functions due to third harmonic generations (hereinafter referred to as THG) and additionally they are applicable for optical switching and optical memory while making use of their optical bistable behavior. Particularly, organic nonlinear optical materials have the following advantages superior over the known inorganic materials. Initially, when compared with crystalline inorganic ferroelectrics such as KDP (potassium dihydrogenphosphate, KH.sub.2 PO.sub.4 and LiNbO.sub.3 (lithium niobate), the organic nonlinear optical materials have larger nonlinear optical coefficients showing the second order nonlinear characteristics. Secondly, when compared with an inorganic semiconductor such as gallium arsenide (Ga--As), the organic nonlinear optical materials have ultrafast response times. Thirdly, different from copper(I) chloride (CuCl) exhibiting a fast response and being capable of operating at room temperature, but being hardly fabricated in a thin film with a thickness of micrometer order, the organic nonlinear optical materials can be easily fabricated into thin films with the thickness of micrometer order. The organic nonlinear optical materials have the possibility of satisfying all requirements which have not been satisfied by the known materials, and thus eager investigations of such materials are continued. For example, the following references disclose organic nonlinear optical materials. J. L. Oudar, J. Chem. Phys., Vol 67, No. 2, pp 446 to 457 (1977), "Optical Nonlinearities of Conjugated Molecules. Stilbene Derivatives and Highly Polar Aromatic Compounds." This reference discloses the results of studies on the second- and third-order hyperpolarizabilities .beta. and .gamma. of 4-dimethylamino-.beta.-nitrostyrene in addition to the derivatives of stilbene. Another reference is G. I. Stegeman and C. T. Seaton, Proceedings of SPIE--The International Society for Optical Engineering, 682, pp 179 to 186 (1986), "Third-order Nonlinear Guided-Wave Optics."
The known third-order nonlinear optical materials include the following two groups of materials. The first group includes .pi.-conjugated polymers, the typical being polydiacetylene, particularly 2,4-hexadiyne-1,6-bis(p-toluenesulfonate) and polyacetylene. The second group includes low molecular weight compounds each having substituents disposed asymmetrically to serve as a donor and an acceptor, the typical being aminonitrostilbene, particularly 4-(N,N-diethylamino)-4'-nitrostilbene. In these compounds, the dimethylamino and diethylamino groups serve as the donors and nitro and cyano groups serve as the acceptors.
The optical nonlinearies of the .pi.-conjugated polymers are based on the polarization of free electrons in the valence electron band, and thus the .pi.-conjugated polymers have the disadvantage resembling that of inorganic semiconductor materials in that the response time is delayed by the resonance effect due to a narrow band gap. Moreover, a .pi.-conjugated polymer that is superior over PTS (2,4-hexadiyne-1,6-bis(p-toluenesulfonate)) has not yet been found.
Since the low molecular weight compounds having asymmetrically disposed substituents serving as a donor and an acceptor exhibit nonlinear optical effects superior over that of 4-(N,N-diethylamino)-4'-nitrostilbene, it have been tried to introduce a longer .pi.-conjugated chain in the compound and to introduce a more active donor-acceptor pair in the compound. However, torsion of the .pi.-conjugated chain is induced as the length of the .pi.-conjugated chain in the compound is increased, leading to the result that the effective length of the .pi.-conjugated chain (i.e. the effective length of delocalized electrons) is decreased. On the other hand, when a more active donor-acceptor pair is introduced in a compound, spontaneous polarization is enhanced leading to the result that the difference in dipole moment between the ground state and the excitation state cannot be increased so large as expected. In addition, deterioration of the material due to absorption of light and delay in response time is accerelated. As the molecular size becomes larger and the spontaneous polarization of the nonlinear optical material becomes extremely enhanced, the solubility in a solvent or a high polymer matrix is lowered and crystallization property is deteriorated to have poor processibility for the preparation of an optical element therefrom.
However, the compounds having substituents disposed asymmetrically and serving as a donor and an acceptor are important organic materials for eager investigations since they exhibit high speed and highly efficient nonlinear optical responses although they have the aforementioned complicated problems.